题干

填写推理理由:

如图,CD∥EF,∠1=∠2,求证:∠3=∠ACB.

证明:∵CD∥EF,

∴∠DCB=∠2 _

∵∠1=∠2,∴∠DCB=∠1. _

∴GD∥CB _

∴∠3=∠ACB _

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答案(点此获取答案解析)

证明:∵CD∥EF,

∴∠DCB=∠2(两直线平行,同位角相等),

∵∠1=∠2,∴∠DCB=∠1(等量代换).

∴GD∥CB(内错角相等,两直线平行).

∴∠3=∠ACB(两直线平行,同位角相等).

故答案为两直线平行,同位角相等;等量代换;内错角相等,两直线平行;两直线平行,同位角相等.

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